The present invention relates to industrial process ovens and, more particularly, to industrial process ovens in which items to be heated are carried on a conveyor therethrough.
The mounting of electronic circuit components on "circuit boards," which provide both positional support for such components and the electrical interconnections therebetween as well as to other portions of the system, is a long established production assembly practice in the providing of electronic circuit products. Over the times such practices have been established, the density of such electronic components has greatly increased on such boards and the circuit interconnections have correspondingly been routed closer to one another as have the leads extending from the various circuit components.
Recently, electronic circuit components, and other circuit components, have come to be mounted on such circuit boards using what is referred to as "surface mount technology." This technology has again resulted in a considerable narrowing of the spacing between leads extending from electronic circuit components intended for such surface mounting, and of the spacing between the circuit interconnections in the circuit boards used therewith. In mounting such surface mount components to metal pads on the circuit boards, solder pastes are placed at the pad locations on which the surface mount circuit component leads are to be soldered in a process something like stenciling. To do so successfully, particle sizes of the tin and lead particles in the solder paste must be made smaller as the spacing between such mounting pads decreases if the paste at each pad is to be suitably confined to the location at which it is intended to be provided.
However, as these particles become smaller, the cumulative surface area of those particles in the paste increases. Heating solder in the presence of oxygen creates oxides on such surfaces of the particles, a result which can hinder soldering because either poor reliability solder joints, or "bridging" between solder joints, will result in the presence of sufficient surface oxidation. Such oxidation depends on the kinds of metals involved, the temperatures reached by those metals during the soldering process, the surface area of the metals that is exposed, and the amount of oxygen present. Because of various requirements for successful soldering, often the only variable that can be controlled is the amount of oxygen present.
In earlier circuit component soldering processes, the amount of oxygen present was not such a significant factor as the greater extent of oxidation occurring in those process was relatively less important. That is because the solder pastes used earlier in those processes contained aggressive fluxes capable of removing such oxides during the soldering process, but such fluxes and other residues had to be cleaned from the resulting soldered circuit boards through the use of solvents. However, currently, there is a strong trend away from such solvent cleaning because of problems presented by such residues and solvents such as unwanted pollution.
In present circumstances, therefore, performing surface mount soldering processes in an inert atmosphere has been found necessary. In an inert atmosphere, such as pure enough nitrogen, oxygen is sufficiently excluded so as to permit a satisfactory soldering outcome. Typically, the inert atmosphere must exhibit a purity such that its oxygen content is less than 50 parts per million, at least during the solder reflow part of the soldering process where temperatures typically come to exceed 150.degree. C.
Such a soldering process has been found convenient to be carried out in an oven formed in conjunction with some kind of a conveying means to provide to the oven the items intended for soldering, and to thereafter remove them from the oven. The use of an oven relatively closed to the outside permits a significant degree of control of the atmosphere therein, as is necessary if a reasonably inert atmosphere is to be provided in the vicinity of the soldering process.
An oven type commonly used for this purpose is an infrared radiation oven. Here, the dominant heating source is an emitter of infrared radiation which is absorbed by an object to be heated as a heat flux, Q, having a value of VE.sub.s A.sub.hi K(T.sub.s.sup.4 -T.sub.hi.sup.4). Here, V is the factor representing the geometric exposure to the source of the items to be heated, E.sub.s is the emissivity of the source, A.sub.hi is the absorptivity of the items to be heated, K is the well-known Stefan-Boltzmann constant, T.sub.s is the source temperature and T.sub.hi is the heated item temperature. Such an oven gives a high heating efficiency and requires a relatively low flow of the gas used in forming the inert atmosphere, a flow just sufficient to remove impurities arising because of volatiles given off during heating of the heated item and the solder paste. Further, the source temperature is easily controlled to give good control over the rate of rise of temperature of the heated item.
On the other hand, where heated items are densely populated circuit boards, some components often will partially "shade" other components with respect to the source thereby reducing the exposure factor for those components. Such shading leads to differential heating among the components, and even between different parts of the same component. Further, if there are substantial mass differences between the components, as there often are in densely populated circuit boards, the components on the board will not heat uniformly requiring the heated item to be exposed for a substantial time to the infrared radiation source if adequate temperature rise is to be assured for the leads of all components. As a result, the rate at which heated items can be conveyed through an infrared radiation dominant oven is relatively slow, a decided disadvantage in a production situation.
Alternatively, a convection heating dominant oven could be used to transfer heat to the items to be heated with a flux of H.DELTA.T where H is the convective film coefficient and .DELTA.T is the temperature difference between the fluid used to transfer heat in the convection heating process and the heated item. The convective film coefficient depends on several variables including velocity of the convective medium flow and the angular direction of that flow with respect to the heated item.
Since the temperature of the convective medium will usually be close to the desired temperature of the heated item, the temperature of the circuit components on the circuit board will not exceed the medium temperature. As a result, larger components can continue to absorb heat while smaller components will not overheat to thereby assure more uniform heating despite substantial mass differences in the components being heated, even in the presence of some components shading others. On the other hand, nitrogen, as the gas of the inert atmosphere, has a low heat capacity which means that high volumes of flow can be required to transfer sufficient heat to the heated items thus risking the possible moving of the electronic circuit components on boards before they adhere thereto, and certainly requiring a large amount of inert medium which can be expensive. Thus, there is desired an oven into which items to be heated can be conveyed and removed, and which will provide relatively uniform heating thereof without encountering prohibitive costs.